THALAMIFLORi?^ 169 



similar effect to that caused by nightfall is mani- 

 fested in intense light, the leaves assuming a vertical 

 position, so that the light rays fall less directly 

 upon them. Plants whose leaves are horizontal 

 suffer more from frost than those the leaves of which 

 are vertical. 



It is the chemical rays that cause the movement. 

 There are motile organs in the short stalk of the 

 leaflets which assist the movement. Leaves from 

 which light is excluded during the day still open their 

 leaflets. When yellow, orange or red rays are shut 

 out the plants behave as under white light. When 

 the violet and blue rays are excluded and only 

 red light present, the plant acts as if it were in the 

 dark. 



The cells are normally turgid, the sap being 

 enclosed by the cell-wall and lining of protoplasm. 

 When the latter is stimulated to change its charac- 

 teristics and allow water to escape, the sleep move- 

 ment is possible. In the leaf tissue are three layers. 

 The upper layer comprises blunt cones, the two 

 lower have stellate cells with inter-cellular spaces. 

 In daylight the chorophyll bodies are arranged to 

 receive as much light as possible. When the light is 

 too intense they collect along the cell-walls, parallel 

 with the rays, so that they receive less light, and 

 collect in groups at the cell corners if the light con- 

 tinues to be intense. 



Wood Sorrel is a common plant in the British Isles 

 as far as the Orkneys, and occurs in Ireland and the 



